Data stored on an optical disc can be read out from the disc by irradiating the rotating disc with a relatively weak light beam with a constant intensity, and detecting the light that has been modulated by, and reflected from, the optical disc.
On a read-only optical disc, information is already stored as pits that are arranged spirally during the manufacturing process of the optical disc. On the other hand, on a rewritable optical disc, a recording material film, from/on which data can be read and written optically, is deposited by an evaporation process, for example, on the surface of a base material on which tracks with spiral lands or grooves are arranged. In writing data on such a rewritable optical disc, data is written there by irradiating the optical disc with a light beam, of which the optical power has been changed according to the data to be written, and locally changing the property of the recording material film.
It should be noted that the depth of the pits, the depth of the tracks, and the thickness of the recording material film are all smaller than the thickness of the optical disc base material. For that reason, those portions of the optical disc, where data is stored, define a two-dimensional plane, which is sometimes called a “storage plane”. However, considering that such a “storage plane” actually has a physical dimension in the depth direction, too, the term “storage plane” will be replaced herein by another term “information layer”. Every optical disc has at least one such information layer. Optionally, a single information layer may actually include a plurality of layers such as a phase-change material layer and a reflective layer.
To read data that is stored on a recordable optical disc or to write data on such an optical disc, the light beam always needs to maintain a predetermined converging state on a target track on an information layer. For that purpose, a “focus control” and a “tracking control” are required. The “focus control” means controlling the position of an objective lens perpendicularly to the information storage plane such that the focus position of the light beam is always located on the information layer. On the other hand, the “tracking control” means controlling the position of the objective lens along the radius of a given optical disc (which direction will be referred to herein as a “disc radial direction”) such that the light beam spot is always located right on a target track.
Various types of optical discs such as DVD (digital versatile disc)-ROM, DVD-RAM, DVD-RW, DVD-R, DVD+RW and DVD+R have become more and more popular these days as storage media on which a huge amount of information can be stored at a high density. Meanwhile, CDs (compact discs) are still popular now. Currently, next-generation optical discs, including Blu-ray disc (BD), which can store an even greater amount of information at a much higher density than any of these optical discs, are under development, and some of them have already been put on the market.
The physical structures of these optical discs change from one type to another. For example, these optical discs are different in physical track structure, track pitch, and depth of the information layer (i.e., the distance from the surface of the optical disc, through which the incoming light enters the disc, to the information layer). To read or write data properly from/on these multiple types of optical discs with those various physical structures, the information layer of each of these optical discs needs to be irradiated with a light beam with an appropriate wavelength by using an optical system that has a numerical aperture (NA) associated with the specific type of the disc.
Recently, an optical disc with two information layers that are stacked in the depth direction has come up as a storage medium with huge storage capacity, and optical disc drives compatible with such optical discs have been put on the market everywhere.
The best conditions for servo controls and servo signals that are required to perform read and write operations on an optical disc vary due to differences in property or characteristic between respective optical disc drives or between optical discs and depending on the temperature conditions during the read or write operation. That is why to perform a read or write operation on an information layer of an optical disc, a so-called “disc loading process”, which is initial adjustment of servo controls and signals, needs to get done following a predetermined procedure.
By performing the disc loading process, the read or write operation can be performed in the best condition on the information layer of the optical disc. However, the greater the number of information layers, the longer it will take to get the disc loading process done and the longer the user will have to wait until the read or write operation can be actually started. This is because the initial adjustment needs to be made on each of those information layers.
Patent Document No. 1 discloses a technique for overcoming such a problem. FIG. 23 is a flowchart showing the procedure of a disc loading process for a dual-layer disc as disclosed in Patent Document No. 1. It should be noted that only a few adjustment-related processing steps are extracted from the disc loading procedure and shown in FIG. 23 and that the timings to turn ON a disc motor or start a focus control could be set arbitrarily.
First, in Step 701 shown in FIG. 23, adjustment is made on the first information layer. Next, in Step 702, a focus jump is done from the first information layer to the second information layer. Then, in Step 703, the result of the adjustment that has been made on the first information layer is set as an initial value of adjustment to be made on the second information layer. Subsequently, in Step 704, using the value that has been set in the previous processing step 703 as an initial value, adjustment is made on the second information layer to end the disc loading process.
According to this disc loading procedure, if the adjustments are made without depending on the properties of the information layers, the result of adjustment on the first information layer will be close to that of adjustment on the second information layer. Therefore, adjustment is made on the second information layer using the result of adjustment on the first information layer as an initial value. Consequently, adjustment can be done on the second information layer in a short time.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2001-319332        